ASTR 1210 (O'Connell) Study Guide

19. EVOLUTION OF THE TERRESTRIAL ATMOSPHERES

Scaled photos of the terrestrial planets.

Why are the atmospheres of the terrestrial planets so astonishingly
different from one another? How have their evolutionary paths
diverged? Given the facts (1) that manmade materials are beginning to
affect Earth's atmosphere and (2) that small changes can make big
differences, this is not merely an academic question. It is essential
to improve our understanding of atmospheric and climate evolution as
quickly as possible.
In the case of the Earth's Moon and Mercury, which have no appreciable
atmospheres, the answer is easy: their gravity is too small to
retain rapidly moving gas molecules near their surface, which therefore
diffuse off into space.
In the case of Venus, Earth, and Mars, we do not yet have a full
understanding of their atmospheric histories, but we have identified
the main natural processes involved and the likely patterns of evolution.
We discuss the evidence that human activities are having a
measurable global effect on the heat content of Earth's atmosphere and
oceans in the final section below.

A. Comparison

VENUS

EARTH

MARS

Relative Planet Mass

0.8

1.0

0.1

Relative Distance from Sun

0.7

1.0

1.5

Relative Atmospheric Mass

100

1.0

0.01

Bulk Atmospheric Composition

CO2

N2, O2

CO2

Relative Water Vapor

0.0001

1.0 [1%]

0.03

Mean Surface Temperature

460oC

20oC

-60oC

There are huge differences in atmospheric mass,
composition, & surface temperature despite a modest range of
distances (only a factor of 2) from the Sun and similar
masses, at least for Venus & Earth.

There is a 10,000:1 range in the mass of the atmospheres of the
three planets; Venus has an atmospheric mass 100 times that of the
Earth despite being a slightly smaller planet. The atmospheric
pressure at the surface of Venus is also 100 times that on Earth (or
the same as the water pressure in a terrestrial ocean at a depth of
3300 feet).
Earth has a nitrogen-oxygen atmosphere, but both Venus and
Mars have carbon dioxide atmospheres.
Earth has a water-rich atmosphere, in which 1% of atmospheric
gases are water molecules. But both Mars and Venus have extremely
dry atmospheres. Mars has proportionately 30x fewer atmospheric
water molecules than Earth; and Venus is 300x poorer in water than
Mars.

There are enormous temperature differences. The
"Goldilocks" syndrome for lifeforms like us: Venus is much too
hot; Mars is too cold; but Earth is "just right".

The habitable zone for Earthlike life is the region
surrounding a star within which liquid water can be stable on
planetary surfaces. Given the temperatures quoted above, in the
case of the Sun the HZ is evidently small: about 0.9-1.4 AU. At
present it contains no planet other than the Earth. The Sun's HZ is
a tiny fraction of the total volume of the Solar System, whose
planetary orbits lie in the range 0.4 to beyond 40 AU.

B. Processes

Many different geophysical processes affect atmospheres, acting to
augment, decrease, or change their contents. Important examples:

Infall of interplanetary material (comets, meteoroids,
asteroids)

This, plus gases captured directly from the solar nebula, was the
source of the earliest atmospheres of the terrestrial planets
but has probably not been important in the last 3.5 billion years.

Volcanic outgassing (see picture above)

Volcanic activity, driven by high internal temperatures, releases
CO2, H2O, SO2, and many other
compounds from the interior into the atmosphere. This was
the dominant source of the present terrestrial
atmospheres.

See
this figure. The
solid symbols show the escape velocities for various planets,
and the dashed lines show molecular velocities at the top of their
atmospheres. Any species whose velocity lies above the symbol
for a given planet will escape to space over time.

Most important molecules have already escaped from the Moon and
Mercury, which are effectively without atmospheres. This is also
true for most of the other small bodies in the solar systems (like the
asteroids & the satellites of the outer planets), though it is easier
to retain an atmosphere if the temperature is lower (as in the case of
Saturn's satellite Titan). Even on the Earth, Venus, and Mars, most
of the light hydrogen and helium molecules have escaped.
The thin but high-speed "solar wind," a continuous outflow of gas from
the Sun, can also strip molecules if it can impinge on the
atmosphere. A
planet's "magnetosphere"
can protect it from the wind, and the Earth, where circulation in the
hot interior maintains a strong magnetic field, is shielded this way. But
Mars' magnetosphere diminished as its interior cooled off and
allowed much of the Martian atmosphere to be stripped off.

"Carbonate-silicate cycle"

See the illustration above. This cycle involves a transfer of carbon
between the Earth's surface, its atmosphere, and its crust.
CO2 vapor is washed out of the atmosphere by liquid
H2O precipitation and is deposited (through chemical
reactions involving silicate rocks) bound into solid limestone
on the seabeds. Deposition over the last billion years has been
assisted by sea animals leaving their shells behind.
Recycling of the crust into the interior melts the rocks
returns CO2 to the atmosphere through volcanic
outgassing.
The net effect of the cycle depends on the relative strengths of the
deposition and return branches. On the Earth, because
the deposition rate is larger than the return rate, this
process has tied up massive amounts of CO2 in
terrestrial surface rocks (about 100 times the current
atmosphere's mass).

Photo-Destruction of H2O

UV radiation dissociates H2O molecules in the upper atmosphere,
allowing H to escape to space (as described above). O2
combines with other molecules, and the water is lost. Since
almost all water is trapped at low altitudes in Earth's atmosphere,
this is not an important process there; but it has had a major
effect on Venus.

The Greenhouse Effect

Absorption of sunlight is the primary source of heat for the
atmosphere and surface of planets. But secondary heating of
the surface and lower atmosphere is caused by the partial trapping
of infrared radiation from the planet's surface by certain gases
(especially H2O, CO2, CH4,
O3). See the diagram above.

Even though these are only "trace gases" in Earth's atmosphere
(for instance, CO2 constitutes only 0.04% of the volume of
the atmosphere), they account for
almost all of the infrared blocking of radiation from the
Earth's surface. Hence, they constitute a powerful "choke-point" in
the flow of atmospheric heat, so they have a major effect on the
temperature balance. For more details on this "Greenhouse Effect,"
see the diagram above (click for enlargement)
and Study Guide 15.

The diagram below shows the
relative heat flows (averaged over a year) for direct and
trapped solar radiation. Note that infrared radiation reflected by
the Greenhouse Effect provides almost twice as much heat input
for the Earth's surface as does direct solar radiation. The large
contribution is due in part to the fact that Greenhouse heating occurs
during all seasons and at night as well as during the day.

C. Equilibrium

The cycle rates for geophysical processes affecting the
atmosphere can be very fast in geological time:

For example, in 50 Myr, which is only 1% of Earth's age...

1 "bar" (= the present mass of Earth atmosphere) of CO2
can be outgassed from Earth's interior

1 bar is 2500x the present CO2
content of Earth's atmosphere.

1 bar of CO2 can be washed out of Earth's atmosphere by
precipitation of CO2-bearing liquid water or ice

1 ocean of H2O can be evaporated & dissociated on Venus

Key concept: the characteristics of an atmosphere are determined by
the balance point or "equilibrium" among all
processes.

If the rates don't balance, there would be relatively rapid evolution
toward more extreme conditions until a new balance is reached.
All significant processes must be considered. In complex
systems like planetary atmospheres it is easy
to overlook important factors.

Feedback mechanisms are critical: they can stabilize the system ("negative
feedback") oraccelerate change ("positive feedback")

Example: the carbonate-silicate cycle provides negative
feedback to help regulate the temperature of Earth's surface.
If the atmospheric temperature rises, there will be increased
evaporation from the oceans, which washes more CO2 from the
atmosphere, which decreases the Greenhouse trapping and hence the
temperature. If the temperature falls, the reverse situation occurs.
This stabilizing feedback occurs only over periods which
are long by human standards (about 400,000 years), however, and
is never able to keep the climate perfectly stable. But it helps
prevent a runaway Greenhouse on the one hand or a permanent ice age
("snowball Earth") on the other.

D. Histories

The existing atmospheres were probably outgassed from the
interior in all cases, amounting to probably 100 bars on Earth
and Venus but less on Mars. An alternative subsidiary source of water
and atmospheric gases: comet impacts.

Earth: "It's the water"....

Earth's distance from the Sun is "just right." A moderate
Greenhouse, driven by the four main Greenhouse gases, elevates the
average surface temperature about 30oC (54oF)
to above the freezing point of water. This much Greenhouse
warming is essential to having a robust biosphere on
Earth.

Earth's mean surface temperature is in the range such that outgassed water
stays liquid near the surface.

There is a "cold trap" at modest altitude in Earth's
atmosphere where the temperature drops below the freezing point of
water. (See this
temperature profile.) Because of the trap, rising water vapor
condenses into droplets or ice crystals and ultimately falls back to
the surface. This prevents escape of water to the stratosphere and
therefore destruction by solar UV radiation. Water is retained near
the surface.

Water washes most CO2 out of the atmosphere,
depositing it in minerals on the ocean floors. The remaining
CO2 constitutes only about 0.04% of the bulk atmosphere
today.

With water vapor condensed into oceans and CO2 scrubbed
from the atmosphere, outgassed nitrogen becomes the dominant
gas in the atmosphere, although its total mass is far smaller
than the Earth's primitive atmosphere.

Liquid H2O provided the environment necessary for
life to develop (about 3-3.5 billion years ago). Organisms
such as blue-green algae capable
of photosynthesis
then processed atmospheric gases to
release
free oxygen (O2). By about 2 billion years ago,
significant amounts of oxygen were present in the atmosphere; the
oxygen density rapidly increased about 500 Myr ago, coinciding with an
explosion in the diversity of lifeforms which thrive on oxygen. Free
oxygen is the most important global tracer of the presence of
Earth-like life. The oxygenation of Earth's atmosphere also
significantly changed the kinds of minerals present on its
surface.

Earth's surface temperature oscillates by about 10o
degrees C (18o F) over periods of about 100,000 years,
driven by the astronomical Milankovitch cycles (see Study Guide 4) but mediated
by the carbonate-silicate feedback cycle. The mean temperature on
Earth over the past 2 million years has been lower than in earlier
epochs.

Venus

Because it is nearer the Sun and had a hotter surface, liquid
H2O deposited by outgassing readily evaporated from its
surface.

Because of the higher temperatures, there was no cold
trap. This allowed water vapor molecules to rise to high
altitudes in the stratosphere, where they were destroyed by
solar UV light.

The absence of the carbonate-cycle cleansing by liquid
H2O precipitation permitted the outgassed CO2
atmosphere to grow.

The end product of the runaway is the massive, hot, dry,
CO2 atmosphere Venus has today.

Continuous volcanic outgassing of materials like sulfur
dioxide in the absence of water cleansing
produced sulfuric acid clouds

Without its abundant water, Earth would probably be like
Venus. In fact, if the Earth as it is today were moved
to Venus' orbit, it would probably evolve quickly toward the
hellish Venusian state.

Mars

Mars was more distant from Sun and therefore colder.

Its smaller mass supported less
outgassing from its interior and therefore a smaller Greenhouse Effect

Early, wet atmosphere & oceans? There is considerable
evidence supporting this interpretation. The water era probably
ended over 1 billion years ago. See Study
Guide 16 and the Mars image page for
more discussion.

An early biosphere? There is some, but marginal,
evidence in favor. See Study Guide
17.

But Mars is a small planet compared to Earth. It has a
large surface area compared to its volume, and therefore
its interior cooled off more quickly. Sufficient
CO2 was not resupplied from the interior; so
the Greenhouse failed. Water eventually froze out and was
presumably deposited in subsurface permafrost.

Atmospheric pressure at the surface is now too low to prevent
the evaporation of liquid water if any appeared.

The end product is a thin, cold, dry atmosphere with only a trace
of water vapor.

Mars presents a harsh surface but one where, with difficulty,
human colonies could live. By contrast, we could never live on Venus.

E. Lessons Learned for Atmospheric Evolution

Little differences can have huge consequences

Our favorable environment is due mainly to our
distance from the Sun and secondarily to the size of our planet.

Biospheres are fragile on Earth-like planets

F. Climate Change: Natural and Unnatural

So far, we have discussed the bulk properties of the
terrestrial atmospheres: mass and composition as they change over many
millions of years. "Climate" refers to the behavior
of surface temperature, precipitation, and wind flow over
the shorter timescales of interest to human beings. Climate
changes on the Earth have major practical consequences, and if,
as appears to be the case, human activities are contributing
to those changes, we need to understand the situation as quickly
as possible.
There are two distinct branches of the study of climate
change: measurements of the temperature and composition
histories of Earth's atmosphere and oceans and modeling of
those histories so that their future properties can be realistically
predicted.

Temperature History

The most conspicuous climate events of the last 2 million years have
been the ice
ages, when a drop in the mean surface temperature allowed
great expansions of the polar ice caps. The last ice age ended about
10,000 years ago. To see the temperature and ice volume histories,
click on the image below. Note that a drop in the mean surface
temperature of only about 3oC was sufficient to
precipitate ice ages.

The small temperature changes that trigger ice ages are importantly
influenced by astronomical
effects (e.g. the "Milankovitch Cycles",
see Study Guide 4) including
small changes in the Earth's orbit and the tilt of its axis, which
affect the amount of insolation in the polar regions.

Intensive studies have also been made of the Earth's temperature
history over the past 1000 years. Except for the period since 1900,
such studies must rely on the use of
various "proxies"
for actual thermometric measures. The profiles show several major
climate events: a "medieval warm period" (about 1000 AD) and
a "little ice age" cold period (about 1600 AD).
But the most important change is a rapid increase in Earth's mean
surface temperature since 1900. Click on the thumbnails below for
enlarged plots of changes in the surface temperature. An
animation of global surface temperature changes on a monthly basis
since 1850 is
shown here.

Surface temperature since 1880

Surface temperature since 800

Remember, we're talking about global surface temperature
here, meaning averages over the entire surface of the Earth,
including the oceans and the southern hemisphere, night and day, and
through four seasons. So our local weather is only a tiny component
of the whole. For instance, despite abnormally cold temperatures in
the eastern US in January 2014, this was globally the fourth warmest
January on record since 1880.
There are large variations in warming depending on geographic
location, as seen in the map below showing worldwide temperature
trends between 1976 and 2000.
Here is a more
recent map showing temperature changes for August 2016 with
respect to mean temperatures for August between 1951 and 1980.

The plots above refer to the temperature in the lowest layers of the
Earth's atmosphere. There are many other geophysical markers
also showing global heating.
Changes
in seven important indicators over 50-100 years are shown
here.
Perhaps of greatest long-term consequence for the climate is the
increasing heat content of the oceans, which absorb about 90% of
the total additional energy. (A layer of ocean water only 11.5 feet
thick contains as much heat as the entire atmosphere.) The chart
below shows the huge rise in ocean heat content that has taken place
since 1960.

The accumulated empirical evidence, from all of these different
indicators, presents an indisputable case for global warming
continuing to the present. And this evidence is no longer disputed,
as it might have been 15 years ago, except for a diminishing group of
committed "warming deniers," mostly non-scientists. One formerly
contrarian group at UC Berkeley has recently independently reanalyzed
surface temperature records and
confirmed the
earlier published trends, as shown above.
The warming has coincided with a rapid increase in the average
atmospheric concentration of carbon dioxide, which was discovered
through
independent spectroscopic studies of atmospheric composition at
Mauna Loa observatory in Hawaii (see left hand panel below). The
present CO2 concentration has no precedent in the recent
geological record: it is 60% higher than the average over the
preceding 600,000 years (and 23% higher than the maximum); see the
right hand panel below.
There is no question that the CO2 increase is due, in turn,
to human use of fossil fuels.

CO2 Concentration since 1960

CO2 Concentration over 600,000 years

Climate Modeling

How are we to interpret these changes, and is there a link between
global heating and human use of fossil fuels and the consequent
rise in CO2 concentration?
The basic physical principles that govern the structure of planetary
atmospheres have been well understood for a century, and the first
prediction that industrialization could lead to sufficient
CO2 production to affect the Earth's temperature balance
through the Greenhouse Effect was actually made 120 years ago
by
Arrhenius.
The problem is that the Greenhouse Effect does not operate in
isolation, and combining it with a myriad of other processes and a
large range of environments to model a system as complex as the real
terrestrial atmosphere is a major challenge. It is necessary to
account for major influences from ocean currents, mountain ranges,
cloud shielding, and solar energy input, among other factors.
A major technical difficulty is that the atmosphere is a strongly
"non-linear" system: output is not simply proportional to
input.

Mathematically, such systems can exhibit "chaotic" behavior,
i.e. divergent results for small changes in the starting point.
This kind of behavior is exemplified by
the "butterfly" effect: a butterfly flapping its wings off the coast
of Africa can, in principle, produce a hurricane over Florida several
weeks later.

Here is a nice BBC video
showing how small disturbances are amplified by atmospheric
instabilities into large storms.

Such complexity makes it very difficult to study the effects that
humans may be having on the atmosphere and climate---and contributes
to the major scientific and political controversies surrounding
"global warming."
Nonetheless, the rapid
(exponential!) growth of the human species (see Study Guide 9) coupled
with our use of technology will inevitably affect Earth's
atmosphere unless we take deliberate actions to avoid this.

There is no doubt that human-induced changes have begun, with the
partial destruction of the ozone (O3) layer, which
shields Earth's surface from solar UV radiation, and the rapid rise
of CO2 content in the atmosphere.

In the absence of any other changes, the added CO2 (double
the pre-industrial amount by mid-21st century) would create
significant additional global warming through the Greenhouse Effect.

Recall that the pre-industrial complement of Greenhouse gases accounted
for a 30o C, or 54o F, increase in Earth's
surface temperature over its "bare" equilibriium temperature. This
much warming was beneficial, and the Earth's surface has long since
adjusted to it.
The problem is that only a small further global temperature
increase (2o C) over a period as short as a century or two
would force a readjustment that would have dramatic effects on
weather, water distribution, and growing seasons on a human scale.

(Two degrees sounds ridiculously small and inconsequential, doesn't
it? But recall that only a 3o C change in global
mean temperature in the other direction could induce an ice age, with
clearly catastrophic consequences for human civilization.)

Fortunately, computer models of the atmosphere and climate change have
rapidly become more sophisticated and realistic as supercomputer power
has accelerated. Most atmospheric physicists agree that the models
are capable of distinguishing human-induced effects from the
atmosphere's continuous natural change. Nonetheless, public
debate has raged over the extent to which a human Greenhouse warming
component is detectable.

The Scientific Consensus

The scientific consensus, based on thousands of studies worldwide
since the 1950's, is that some human-induced warming has
occurred (probably at least 50% of the temperature rise
over the last 60 years) and that significant additional warming is
expected over the next 100 years. The conclusion of the 2014 United
Nations Panel on Climate Change was that it is "extremely likely" that
humans are the main cause of climate warming since 1950.
Here is a 2013 summary of the situation from the American Geophysical
Union:

"Human activities are changing Earth's climate. At the global level,
atmospheric concentrations of carbon dioxide and other heat-trapping
greenhouse gases have increased sharply since the Industrial Revolution.
Fossil fuel burning dominates this increase. Human-caused increases in
greenhouse gases are responsible for most of the observed global average
surface warming of roughly 0.8C (1.5F) over the past 140 years.
Because natural processes cannot quickly remove some of these gases
(notably carbon dioxide) from the atmosphere, our past, present, and
future emissions will influence the climate system for millennia."
"Extensive, independent observations confirm the reality of global
warming. These observations show large-scale increases in air and sea
temperatures, sea level, and atmospheric water vapor; they document
decreases in the extent of mountain glaciers, snow cover, permafrost,
and Arctic sea ice. These changes are broadly consistent with
long-understood physics and predictions of how the climate system is
expected to respond to human-caused increases in greenhouse gases. The
changes are inconsistent with explanations of climate change that rely
on known natural influences."
"Impacts harmful to society, including increased extremes of heat,
precipitation, and coastal high water are currently being experienced,
and are projected to increase. Other projected outcomes involve threats
to public health, water availability, agricultural productivity,... and coastal
infrastructure, though some benefits may be seen at some times and
places....While important scientific uncertainties remain as to which particular
impacts will be experienced where, no uncertainties are known that could
make the impacts of climate change inconsequential. Furthermore,
surprise outcomes, such as the unexpectedly rapid loss of Arctic summer
sea ice, may entail even more dramatic changes than anticipated."

Here is a historical perspective on the predicted impacts of global
warming.
Such conclusions have been disputed by the fossil fuel industries and
their political allies and by a small subset of climate scientists.
Very few of the publicly-prominent "climate skeptics" have the
qualifications needed to assess the evidence based on original sources
--- i.e. the scientific literature. Instead, most are simply echoing
talking points which they think might gain the most traction with
public opinion, even if those were demolished long ago. Meanwhile,
the scientifically-qualified skeptics are in retreat on the question
of the existence of warming, and some are accepting the likelihood of
human involvement in global warming.
In assessing the controversy, it's useful to remember that scientists
do not easily reach a consensus. There are tremendous incentives for
scientists to contravene the "conventional wisdom," to be able to
demonstrate convincingly that their peers are misguided. That's how
scientists become famous. No one earns great credit for merely
confirming what people already know. If scientists have reached a
consensus in the case of global warming, this means that contrary
evidence is unconvincing both in quality and quantity.
What to do? There is no doubt that humans can adjust to whatever
changes occur over 100 years...so we will survive. But the robustness
of our economy depends on the stability of climate patterns,
not variations in them. The costs of dislocations produced by
major climate change could be enormous. Hurricanes Katrina (2005)
and Sandy (2012) and the three destructive 2017 storms are
good examples of the scale of the local economic disruptions
that climate change could produce, even though it is difficult to
determine the extent to which those storms were intensified by such
change. Such dislocations could easily favor nations other than
the US (the southwestern quarter of which, for instance, could
suffer severe drought), so climate change becomes important to
our national economic
security.
The prudent course is to take steps to reverse the
increase in Greenhouse gases until there is a better understanding
of what we are doing to the atmosphere.
Reading for this lecture: